The invention relates to adjustable telescopic devices of the type in which an inner tube is slidably inserted in an outer tube and extends therefrom and is adapted to transmit an axial load to the outer tube, the inner tube being lockable in a plurality of positions relative to the outer tube. The invention has been developed especially for use in spindles for working chairs having an adjustable height, wherein the weight of the user in addition to the weight of the seat and possibly the back of the chair provides the load on the inner tube. However, it is feasible to use the invention in a number of other environments, for instance as adjustable table legs, shuttering supports, braces for agricultural machines etc., wherever there is a need for an adjustment of a height or a length.
The possibility of adjusting the height of working chairs is very important in many situations, especially when the same work-table is used by different persons at different times, a correct seating level being essential for the comfort of the user. For such a possibility to be regularly used when needed it is absolutely necessary that the user can operate the device quickly and from a seated position, even when having no special technical skill.
For this purpose it is usual to use a telescopic device of the type referred to initially. A problem in this connection is to combine the possibility of easily operating the device with a positive locking in the chosen position. In many such telescopic devices the locking is provided by means of some sort of wedging action provided by axially movable, spring loaded wedges which either force flaps partly cut out from the inner tube radially outwardly against the outer tube or force wedges into the space between the inner and the outer tube. However, unless the telescopic movement is braked in some other way, the locking device must take the full axial load, for instance the weight of a relatively heavy person, and a high locking force is then required. In this respect a difficulty resides in the fact that a small wedge angle, although providing a secure locking, is also heavy to release, whereas a locking using larger wedge angles is less secure. The use of wedge elements between the outer tube and downwardly converging surfaces on the inner tube is referred to already in German Patent Specification 130 366 from 1901, but this patent specification also describes and illustrates another possibility, namely to replace the wedge elements with balls which are actuated from below by a spring loaded disc. In such a device the locking is performed by the balls wedging due to the spring load, the locking becoming firmer the larger the weight with which the seat is loaded, whereas for adjustment movements in released position of the disc only rolling friction has to be overcome. However, an inherent drawback is that the balls when heavily loaded by the weight of the user, will exert a heavy local load on the outer as well as the inner tube, with the result that these tubes may deform and gradually become permanently damaged, with the consequence that the function becomes unsatisfactory and operation of the device from a seated position is no longer possible.